The present invention relates to an improvement in output control of a charger (a transfer electrode and a separation electrode) in an electrophotographic copying apparatus, and in particular, to that in output control of a transfer electrode.
Generally, in a copying machine of the type mentioned above, a document placed on a platen glass is illuminated by light of a luminous lamp of a reading section, then, reflected light from the document forms an electrostatic latent image on a photoreceptor (OPC (organic photoreceptor) or the like) drum or a belt charged electrically at high voltage, the electrostatic latent image is developed to be a visible toner image in a developing section, and the toner image is transferred onto a transfer sheet entering a transferring/separating section synchronously with the photoreceptor and the transfer sheet is separated from the photoreceptor to be conveyed to a fixing section where the toner image is fused and fixed through heat fusing on the transfer sheet which is finally collected.
However, when the humidity is high in the place where a copying machine is installed, a resistance value of a transfer sheet is varied due to its moisture absorption. Therefore, even when a stationary electric current at high voltage is given to a transfer electrode, electric charges different in terms of polarity from that in toner which are generated by the high voltage current given do not remain on the transfer sheet but leak along it through grounding, resulting in a transfer failure wherein an image is not transferred normally thus the toner image on the photoreceptor is conveyed to a cleaning section where the toner image is erased fruitlessly. Or, when a high voltage current is intensified for the purpose of correcting the aforementioned failure, the percentage of moisture content of electric charges generated by the high voltage current emerged from the transfer electrode rises and the charges begin to float on the transfer sheet whose electric resistance has been lowered, resulting in blurred transfer wherein the toner image on the photoreceptor is blurred to be transferred onto the transfer sheet.
As a technology for preventing or eliminating the phenomenon mentioned above, Japanese Patent Publication Open to Public Inspection No. 57042/1978 (hereinafter referred to as Japanese Patent O.P.I. Publication) discloses a technology wherein a humidity is measured in a copying machine and is fed back to a power supply for a transfer electrode, Japanese Patent O.P.I. Publication No. 28081/1980 discloses a technology wherein a resistance value of each transfer sheet is measured and is fed back to a power supply for a transfer electrode, and Japanese Patent O.P.I. Publication No. 152267/1980 discloses a technology wherein a high humidity condition is detected and thereby a high voltage current for a charging electrode is controlled.
However, measurement of a resistance value of each transfer sheet brings about disadvantages that a large-scaled mechanism is required and a transfer sheet jam is caused, which is not preferable.
In a technology wherein a humidity is measured and is fed back to a current for transferring or the like, on the other hand, a correlation between a humidity and a current for transferring is fixed equally after measurement of a humidity, leaving the number of sheets stacked out of consideration. Therefore, when transfer sheets in quantity of not less than 20 are conveyed for copying under the condition of a high humidity, a phenomenon of an image quality problem of blurred transfer is caused by an excessive current, which has been a problem.